The invention relates to aluminum alloys containing lithium as an alloying element, and particularly to a process for stretching the aluminum-lithium alloys without producing strain-induced imperfections known as Luder lines.
It has been estimated that some current large commercial transport aircraft may be able to save from 15 to 20 gallons of fuel per year for every pound of weight that can be saved when building the aircraft. Over the projected 20-year life of an airplane, this savings amounts to 300 to 400 gallons of fuel for every pound of weight saved. At current fuel costs, a significant investment to reduce the structural weight of the aircraft can be made to improve overall economic efficiency of the aircraft.
The need for improved performance in aircraft of various types can be satisfied by the use of improved engines, improved airframe design, or by the use of new or improved structural materials. Improvements in engines and aircraft design have been vigorously pursued, but only recently has the development of new and improved structural materials received commensurate attention, and their implementation in new aircraft designs is expected to yield significant gains in performance.
Materials have always played an important role in dictating aircraft structural concepts. Since the early 1930's, structural materials for large aircraft have remained remarkably consistent, with aluminum being the primary material of construction in the wing, body and empennage, and with steel being utilized for landing gears and certain other specialty applications requiring very high strength. Over the past several years, however, several important new material concepts have been under development for incorporation into aircraft structures. These include new metallic materials, metal matrix composites and resin matrix composites. It is believed by many that improved aluminum alloys and carbon fiber resin matrices will dominate aircraft structural materials in the coming decades. While composites will be used in increased percentages as aircraft structural materials, new lightweight aluminum alloys, and especially aluminum-lithium alloys show great promise for extending the usefulness of materials of this type.
Heretofore, aluminum-lithium alloy products of the types described hereinafter have not been used in aircraft structure. Aircraft applications for alloys of the type have heretofore been restricted to uses wherein the mill product has been adapted by machining or otherwise contouring the product form without the need for stretching. The state-of-the-art in producing suitably strong, yet damage-tolerant aluminum lithium alloy sheets, has progressed to a point that its inherent properties are attractive for air transport body skins. Body-skin applications, however, have been restricted because of the alloys' propensity to form Luder-lines at low relative amounts of contour stretching. These Luder lines are aesthetically objectionable, and may compromise engineering properties.
It is generally understood that Luder line phenomena are associated with non-homogeneous deformation of the metal alloy. Although other aluminum-based alloy materials exist that only occasionally suffer from the formation of Luder lines, lithium additions to aluminum provide a substantial density reduction which has been determined to be very important in decreasing the overall structural weight of the aircraft. While substantial strides have been made in improving the aluminum-lithium processing technology, a major challenge remains to obtain a stretch-formed sheet of these aluminum-lithium alloys whose surfaces are substantially free of Luder lines.